First and second sets of quadrature-amplitude-modulation (QAM) symbols transmitted parallelly in time can differ in the respective patterns of labeling lattice-points in the two sets of QAM symbol constellations, which constellation rearrangement approach provides “labeling diversity”. Labeling diversity can lessen the error in reception of transmitted data accompanied by noise, as compared to that in which the same pattern is used to label lattice-points of the first and second sets of QAM symbols transmitted parallelly in time and demodulated separately to recover respective soft-bit demodulation results. Akram Bin Sediq and Halim Yanikomeroglu described a technique for soft combining soft-bit demodulation results in a paper titled “Performance Analysis of Soft-Bit Maximal Ratio Combining in Cooperative Relay Networks”, which was published in IEEE Transactions on Wireless Communications (Volume: 8, Issue: 10, October 2009), pp. 4934-4939. This paper referred to this soft combining technique by the name “soft-bit maximal-ratio combining” or by its abbreviation “SBMRC”.
Panasonic Corporation sent a paper titled “Enhanced HARQ Method with Signal Constellation Rearrangement” to the TSG-RAN Working Group 1 for discussion during its Meeting #19 held Feb. 27-Mar. 2, 2001 in Las Vegas, Nev., USA. Combining two 16QAM transmissions with labeling diversity between the labeling of the lattice points in their respective square 16QAM symbol constellations was reported to provide a 1.2 dB advantage over Chase combining two 16QAM transmissions without labeling diversity. Combining two 64QAM transmissions with labeling diversity between the labeling of the lattice points in their respective square 64QAM symbol constellations was reported to provide a 1.8 dB advantage over Chase combining two 64QAM transmissions without labeling diversity. Turbo coding rate was ¾, both for 16QAM transmissions and for 64QAM transmissions. The Panasonic Corporation paper was not directed to COFDM signals.
In Gray mapping of QAM, the plural-bit labels of immediately adjacent lattice-points differ in only a single one of their bits. With regard to Gray mapping, it has been shown that a constellation rearrangement approach improves the performance if two or more versions of the same word are transmitted. The constellation rearrangement scheme for Gray mapping is based on different levels of reliability for the bits, depending on the position of the selected 16QAM symbols within the constellation. Consequently, the rearrangement rules focus on changing the location of the rearranged version of the 16QAM symbol to achieve an averaging effect of the levels of reliability. First and second sets of 16QAM symbols transmitted parallelly in time are labeled such that the labeling of each set of 16QAM symbols bits more likely to experience error in the labeling of each set of 16QAM symbols is in accordance with the bits less likely to experience error in the labeling in the other set of 16QAM symbols. For details on constellation rearrangement for 16-QAM Gray mapping, one is referred to U.S. Pat. No. 7,920,645 titled “Data transmissions in a mobile communication system employing diversity and constellation rearrangement of a 16 QAM scheme” granted 5 Apr. 2011 to Alexander Golitschek Edler Von Elbwart, Christian Wengerter and Isamu Yoshi. U.S. Pat. No. 7,957,482 titled “Bit-operated rearrangement diversity for AICO mapping” granted 7 Jun. 2011 to Alexander Golitschek Edler Von Elbwart, Christian Wengerter and Isamu Yoshi describes more extensively the use of labeling diversity for more than one set of 16QAM symbols transmitted parallelly in time. (AICO is the acronym for “Antipodal Inverted Constellation”.) The Von Elbwart et alii patents were not directed to COFDM signals.
In June 2005 a paper “Symbol mapping diversity design for multiple packet transmissions” authored by Harvind Samra, Zhi Ding, Peter M. Hahn was published in IEEE Transactions on Communications Vol. 53, No. 5, pp. 810-817. The Samra et al. paper presented a simple, but effective method of enhancing and exploiting diversity from multiple packet transmissions in systems that employ nonbinary linear modulations such as phase-shift keying (PSK) and quadrature amplitude modulation (QAM). This diversity improvement results from redesigning the symbol mapping for each packet transmission. Symbol mapping diversity (SMD) requires a small increase in receiver complexity, but provides very substantial reductions of bit error rate when applied to additive white Gaussian noise (AWGN) and flat-fading channels. The general SMD concept was later incorporated in multiple-input/multiple-output (MIMO) and multiple-input/single-output (MISO) communication systems, but was referred to as “labeling diversity” by Maciej Krasicki in his paper “The essence of 16-QAM labeling diversity” published 11 Apr. 2013 in Electronics Letters, Vol. 49, issue 8, pp. 567-569.
A 2015 paper “Labeling Diversity for 2×2 WLAN Coded-Cooperative Networks” authored by Saqib Ejaz, Feng-Fan Yang and Hong-Jun Xu was published in Radio Engineering, Vol. 24, No. 2, pp. 470-479. Wireless local area networks (WLAN) utilize OFDM signals, and this Ejaz et alii paper considers labeling diversity in QAM of OFDM signals employed in MIMO networks. This Ejaz et al. paper does not propose the application of labeling diversity to QAM of respective sets of OFDM subcarriers within a single COFDM DCM signal. This Ejaz et alii paper does aver that the general idea of labeling diversity can be extended to other high order modulation schemes besides 16QAM. This Ejaz et al. paper reports that labeling diversity has shown promising BER performance improvements in systems without labeling diversity, and that labeling diversity also lowers the Error Floor (EF) region by ensuring error-free feedback during the iterative decoding process.
In the following portion of this specification, in its accompanying drawings and in its accompanying claims the lower-frequency and higher-frequency halves of the complete frequency spectrum of a COFDM signal are respectively referred to in shortened form simply as its “lower sideband” and “upper sideband”. The lower and upper sidebands of a COFDM modulated signal that convey the same coded data mirror each other in double-sideband COFDM (abbreviated as DSB-COFDM). The term asymmetric-sideband COFDM (abbreviated as ASB-COFDM) is used herein to specify COFDM in which the lower and the upper sidebands of a COFDM signal convey the same coded data, but do not mirror each other. DSB-COFDM and ASB-COFDM are respective species of COFDM dual-subcarrier-modulation (DCM), which may be referred to as DCM-COFDM. DCM-COFDM uses pairs of OFDM subcarriers, the OFDM subcarriers in each of those pairs conveying the same coded data.
In some species of ASB-COFDM the OFDM subcarriers in each pair of them are separated a uniform distance from each other so as to fall in the lower and the higher halves of frequency spectrum respectively. Such separation improves reliability of reception, especially when there are narrow-band interferences. Such species of COFDM DCM are disclosed in patent application US-20170104553-A1 published 13 Apr. 2017, titled “LDPC Tone Mapping Schemes for Dual-Sub-Carrier Modulation in WLAN” and claiming an original filing date of 11 Oct. 2016 for inventors Jian-Han Liu, Sheng-Quan Hu, Tian-Yu Wu and Thomas Edward Pare, Jr. US-20170104553-A1 describes respective mappings of the sets of 16QAM symbols transmitted parallelly in time, which mappings are similar to each other.
Single-sideband COFDM or (SSB-COFDM) modulation of radio-frequency (RF) signals has been used several years for over-the-air broadcasting of DTV in accordance with the DVB-T and DVB-T2 Standards for Digital Video Broadcasting in several countries other than the United States of America and Canada. SSB-COFDM RF signals are now being broadcast in the Republic of South Korea and in the United States of America in accordance with an ATSC 3.0 Standard developed by the Advanced Television Systems Committee, an industry-wide consortium of DTV broadcasters, manufacturers of DTV transmitter apparatus, and manufacturers of DTV receiver apparatus.
In DSB-COFDM the lower and upper halves of the frequency spectrum of the COFDM signal mirror each other. Prior-art receivers for DSB-COFDM RF signals, such as receivers for DTV broadcasting, have folded the frequency spectrum in half by synchrodyning to baseband before applying discrete Fourier transform (DFT) and demapping the resultant quadrature amplitude-modulation (QAM) of COFDM signal subcarriers. The constructive combining of mirrored OFDM subcarriers improves the signal-to-noise ratio (SNR) of reception over an additive-white-Gaussian-noise (AWGN) channel by 3 dB. Receivers that demodulate DSB-COFDM RF signals using either single-sideband (SSB) or asymmetric-sideband (ASB) techniques are described in U.S. patent application Ser. No. 15/641,014 filed by Allen LeRoy Limberg on 3 Jul. 2017, titled “Double-sideband COFDM signal receivers that demodulate unfolded frequency spectrum” and published 1 Feb. 2018. Limberg prescribed individual discrete Fourier transform (DFT) of the lower and upper halves of the frequency spectrum of the COFDM modulation signal and demapping the resulting sets of QAM symbols from those two halves of that frequency spectrum, then diversity combining their corresponding QAM-lattice-point labels. Maximal-ratio combining soft bits of corresponding QAM-lattice-point labels improves SNR of reception over an AWGN channel by 5.5 dB, irrespective of shaping gain. This 2.5 dB better SNR is in line with observations concerning multiple-in/multiple-out (MIMO) reception of COFDM modulation signals from plural-antenna arrays, as reported in U.S. Pat. No. 7,236,548 titled “Bit level diversity combining for COFDM system” issued 26 Jun. 2007 to Monisha Ghosh, Joseph P. Meehan and Xuemei Ouyang. (DSB-COFDM modulation affords some frequency diversity that can help receivers as described in U.S. patent application Ser. No. 15/641,014 to overcome some frequency-selective fading and narrowband interference, so long as neither affects the more central frequencies of the transmission channel.)
U.S. patent application Ser. No. 15/796,834 titled “Communication systems using independent-sideband COFDM” filed 29 Oct. 2017 by Allen LeRoy Limberg was published 3 May 2018 as US-2018-0123857-A1. It describes respective symbol constellation arrangements of QAM subcarriers in the lower and upper sidebands of a COFDM signal differing from each other. Dissimilarity in the respective mapping patterns in the two sets of QAM symbols transmitted parallelly in time is referred to as “labeling diversity” and is described being done to obtain “shaping” gain in addition to diversity gain. US-2018-0123857-A1 also describes respective symbol constellation arrangements of QAM subcarriers in the lower and upper sidebands of a COFDM signal differing from each other, so as to reduce the peak-to-average-power ratio (PAPR) of that COFDM signal.
U.S. patent application Ser. No. 16/039,259 titled “COFDM DCM Communication Systems with Preferred Labeling-Diversity Formats” filed 18 Jul. 2018 by Allen LeRoy Limberg and published 2 May 2019 as US-2018-0132171-A1 describes labeling diversity (sometimes referred to as “symbol recombination”) designed to support a soft-bit maximal-ratio combining (SBMRC) procedure. Bits more likely to experience error in lattice-point labels of the mapping pattern for the first set of symbol constellations correspond to bits less likely to experience error in lattice-point labels for the second set of symbol constellations. Bits less likely to experience error in lattice-point labels of the mapping pattern for the first set of symbol constellations correspond to bits more likely to experience error in lattice-point labels for the second set of symbol constellations. The SBMRC procedure is implemented in a receiver by diversity combining pairs of corresponding labels from the first and second sets of symbol constellations. Receivers for two sets of QAM symbols transmitted parallelly in time can thus be designed to exploit labeling diversity to achieve shaping gain significantly larger than that which can be secured from NuQAM—i.e., QAM with non-uniform spacing between labeled points in the QAM symbol constellation mapping. NuQAM offers an SNR gain for the additive white Gaussian noise (AWGN) channel that can quite closely approach the ultimate 1.53 dB limit for geometric shaping gain posited by Shannon, but cannot exceed that limit. Also, NuQAM requires forward-error-correction (FEC) code rates not significantly greater than ½ in order to provide shaping gain, while SBMRC provides significant shaping gains at higher FEC code rates.
In the past, broadcasters' primary concern with high PAPR of COFDM signal was its costing expensive power bills for linear power amplification in the transmitter. Newer designs of COFDM transmitters for broadcast television improve power amplifier efficiency, by using variants of the methods described in U.S. Pat. No. 6,625,430 titled “Method and apparatus for attaining higher amplifier efficiencies at lower power levels” granted 23 Sep. 2003 to Peter J. Doherty. Accordingly, PAPR reduction techniques have become less likely to be resorted to. However, the large PAPR of COFDM also causes problems in receiver apparatus that are not avoided and indeed may be exacerbated by using a Doherty method in the broadcast transmitter. These problems concern maintaining linearity in the radio-frequency (RF) amplifier, in the intermediate-frequency (IF) amplifier (if used) and in the analog-to-digital (A-to-D) converter.
U.S. Pat. No. 8,040,963 titled “Method for reducing peak-to-average power ratio in an OFDM transmission system” claiming a 20 Oct. 2006 priority date was granted 18 Oct. 2011 to Ondrej Hlinka, Ondrej Hrdlicka and Pavol Svac. The patent describes PAR reduction based on a complementary parity coding in which the coding rules are derived from an appropriate auto-correlation property of transmitted symbol sequences. The techniques were also described by P. Svac and O. Hrdlicka in a paper titled “A high peak-to-average power ratio reduction in OFDM systems by ideal N/2-shift aperiodic auto-correlation property” presented as part of the Joint IST Workshop on Mobile Future, 2006 within the Symposium on Trends in Communications '06 held 24-27 Jun. 2006 in Bratislava, Slovakia. This paper and U.S. Pat. No. 8,040,963 assert that a significant PAPR reduction of 6 dB, independent of the number of subcarriers, can be achieved in OFDM by assuring the appropriate auto-correlation property of transmitted data symbol sequences. Binary phase-shift keying (BPSK) data symbols were arranged in paired sequences, each successive pair of sequences being transmitted in a respective OFDM symbol.
COFDM can use a technique symmetric cancellation coding (SCC) in which OFDM carriers are arranged in pairs, the QAM of each of the two OFDM carriers in a pair being antipodal to the QAM of the other. While such SCC has been used principally implementing intercarrier interference (ICI) cancellation, it is reported to reduce PAPR of COFDM in a paper titled “Analysis of Coherent and Non-Coherent Symmetric Cancellation Coding for OFDM Over a Multipath Rayleigh Fading Channel” Abdullah S. Alaraimi and Takeshi Hashimoto presented at the IEEE 64th Vehicular Technology Conference held 25-28 Sep. 2006 in Montreal, Quebec, Canada. Alaraimi and Hashimoto's simulations using 2-dimensional modulation of OFDM subcarriers found 0.5 dB lowering of the PAPR of COFDM when SCC was employed. The particular size of the COFDM modulation constellations employed in the simulations was not specified in this paper.
Significantly greater lowering of the PAPR of COFDM is obtained from labeling diversity other than that provided by SCC, according to a paper titled “PAPR Performance of Dual Carrier Modulation using Improved Data Allocation Scheme” that Soobum Cho and Sang Kyo Park presented at the 13th International Conference on Advanced Communication Technology (ICACT 2011) held 13-16 Feb. 2011 in Seoul, Republic of Korea. Their dual carrier modulation (DCM) spaces the OFDM subcariers N/2 carriers apart to maximize frequency diversity, N being the total number of carriers in the OFDM signal. FIG. 4 of that paper shows PAPR of OFDM DCM being about 2.5 dB less than PAPR of conventional OFDM, when 16QAM of OFDM subcarriers is used. The two 16QAM mapping patterns Cho and Park used to secure lower PAPR were described earlier by Martin Geoffrey Leach and Peter Anthony Borowski in a patent application titled “Signal decoding systems” and published 4 Sep. 2008 as US-2008-0212694-A1.
Superposition coded modulation (SCM) is described in detail by Li Peng, Jun Tong, Xiaojun Yuan and Qinghua Guo in their paper “Superposition Coded Modulation and Iterative Linear MMSE Detection”, IEEE Journal on Selected Areas in Communications, Vol. 27, No. 6, August 2009, pp. 995-1004. In the SCM these authors particularly describe, the four quadrants of square 16QAM symbol constellations are each Gray mapped independently from the others and from the pair of bits in the map label specifying that quadrant. Peng et alli studied iterative linear minimum-mean-square-error (LMMSE) detection being used in the reception of SCM and found that SCM offers an attractive solution for highly complicated transmission environments with severe interference. Peng et alli analyzed the impact of signaling schemes on the performance of iterative LMMSE detection to prove that among all possible signaling methods, SCM maximizes the output signal-to-noise/interference ratio (SNIR) in the LMMSE estimates during iterative detection. Their paper describes measurements that were made to demonstrate that SCM outperforms other signaling methods when iterative LMMSE detection is applied to multi-user/multi-antenna/multipath channels.
Jun Tong and Li Peng in a subsequent paper “Performance analysis of superposition coded modulation”, Physical Communication, Vol. 3, September 2010, pp. 147-155, separate superposition coded modulation into two general classes: single-code superposition coded modulation (SC-SCM) and multi-code superposition coded modulation (MC-SCM). In SC-SCM the bits in the superposed code layers are generated using a single encoder. SC-SCM can be viewed as conveying a special BICM scheme over successive SCM constellations. In MC-SCM the bits in the superposed code layers are generated using a plurality of encoders supplying respective codewords. MC-SCM can be viewed as conveying special-case multi-level coding (MLC) scheme over successive SCM constellations.
Single carrier modulation is referred to as “SCM” in some texts other than this, but hereafter in this document the acronym “SCM” will be used exclusively to refer to superposition coded modulation. SCM can be used to convey a single codestream, rather than more than one codestream. The forms of mapping used for square QAM symbol constellations are the principal concern in the invention treated in this text, not the conveying of a plurality of codestreams concurrently. Accordingly, embodiments of the invention specifically described herein employ SC-SCM.
The above-referenced Limberg U.S. patent application Ser. No. 16/039,259 titled “COFDM DCM communication systems with preferred labeling-diversity formats” disclosed various forms of COFDM DCM signal. In certain of these forms of COFDM DCM signal the quadrature amplitude modulation (QAM) of COFDM subcarriers is Gray mapped to position palindromic lattice-point labels along one of the diagonals of each square QAM constellation. A palindrome is a word that reads the same both forward and backward. A palindromic lattice-point label is one that exhibits the same successive bit pattern reading from right to left as reading from left to right. There are four palindromic lattice-point labels in a square 16QAM symbol constellation, namely: 0000, 0110, 1001 and 1111. There are eight palindromic lattice-point labels in a square 64QAM symbol constellation, namely: 000000, 001100, 010010, 011110, 100001, 101101, 110011 and 111111. U.S. patent application Ser. No. 16/039,259 also discloses square 16QAM constellations in which the four palindromic lattice-point labels 0000, 0110, 1001 and 1111 are positioned in respective corners in each of the 16QAM symbol constellations in a first set of them and are clustered together around the center of each of the 16QAM symbol constellations in a second set of them. This can be achieved by using superposition coded modulation—i. e., by SCM mapping of lattice point labels of lattice points in the 16QAM constellations. U.S. patent application Ser. No. 16/039,259 also revealed that soft-bit maximal-ratio combining (SBMRC) can be applied not just to Gray mapping, but also to SCM mapping.
There is a wide amount of prior art which retrospectively considered is pertinent to improvements over “single-sideband” COFDM for radio-frequency transmissions in single-channel applications such as digital television (DTV) broadcasting. However, there appears to have been no “road map” to guide skilled systems designers for such single-channel applications in selecting from the wide amount of such prior art in other communication systems to optimize single-channel radio-frequency transmissions. The need for such a “road map” is evidenced by the recently adopted ATSC 3.0 Standard for DTV Broadcasting using decades-old “single-sideband” COFDM technology, despite the Advanced Television Systems Committee comprising experts in design of the various components of DTV systems charged with selecting the best-of-the-best known technology for the newer standard.